Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability, etc. The various fluoropolymers are, for example, described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997. The fluoropolymers may have a partially fluorinated backbone, generally at least 40% by weight fluorinated, or a fully fluorinated backbone. Particular examples of fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF).
In particular PTFE dispersions have found wide applications because of the unique and desirable chemical and physical properties of PTFE. For example, PTFE dispersions are frequently used in preparing coating compositions for coating metal substrates such as cookware because of the high temperature resistance of PTFE and anti-stick properties. The chemical resistance and corrosion resistance of PTFE have been exploited in industrial applications such as in chemical manufacturing plants. Because of its unrivaled weathering stability, PTFE has further been used for coating woven glass cloth for architectural fabrics. Details on fabrication and processing of PTFE-dispersions can be found in Fluoroplastics, Vol. 1, “Non-melt processible fluoroplastics”, p. 168-184, Sina Ebnesajjad, Plastics Design Library, Norwich, N.Y., 2000.
Aqueous dispersions of PTFE are typically obtained by aqueous emulsion polymerization. The aqueous emulsion polymerization is typically carried out in the presence of a fluorinated surfactant. Frequently used fluorinated surfactants include perfluorooctanoic acids and salts thereof, in particular ammonium perfluorooctanoic acid. Further fluorinated surfactants used include perfluoropolyether surfactants such as disclosed in EP 1059342, EP 712882, EP 752432, EP 816397, U.S. Pat. No. 6,025,307, U.S. Pat. No. 6,103,843 and U.S. Pat. No. 6,126,849. Still further surfactants that have been used are disclosed in U.S. Pat. No. 5,229,480, U.S. Pat. No. 5,763,552, U.S. Pat. No. 5,688,884, U.S. Pat. No. 5,700,859, U.S. Pat. No. 5,804,650, U.S. Pat. No. 5,895,799, WO 00/22002 and WO 00/71590. The aqueous emulsion polymerization method to produce PTFE is well known and is described in for example U.S. Pat. No. 2,434,058, U.S. Pat. No. 2,965,595, DE 25 23 570 and EP 030 663.
Core-shell polymerization of PTFE has also been described to improve certain properties of PTFE. For example, core-shell polymerization of PTFE is described in U.S. Pat. No. 2,965,595, U.S. Pat. No. 3,142,665 and EP 525 660. EP 481 509 discloses a core shell polymerization to obtain PTFE that has good blending and dispersing properties in a resin, an elastomer, or a paint. It is taught in this patent that the shell of the PTFE particles should have a molecular weight of about 10,000 to 800,000 g/mol. A chain transfer agent is typically used to achieve the low molecular weight in the shell of the core shell particles.
WO 02/072653 teaches core shell PTFE dispersions that contain at least 1.5% by weight of rod-shaped PTFE particles that have a length to diameter ratio of at least 5. The majority of PTFE particles in the dispersion are cylindrical, i.e., having a length to diameter ratio of 1.5 or more. It is taught that such dispersions have an enhanced shear stability combined with a high critical cracking thickness. However, to produce non-spherical particles the polymerization conditions need to be carefully controlled. Moreover, WO 02/072653 teaches the use of a telogenic agent during the last stage of the polymerization, which will result in producing a substantial amount of very low molecular weight PTFE and which might potentially compromise the desirable properties of PTFE.
U.S. Pat. No. 4,326,046 teaches a core-shell polymerization of TFE to produce PTFE particles that have in their sheath layers modifiers that have ionic groups. It is taught that such modified PTFE particles can be used in making cation exchange membranes. The amount of the modifier may be up to 30% by weight.
For use in making coating compositions, the dispersion is generally upconcentrated following the aqueous emulsion polymerization to yield the desired fluoropolymer solids, which is typically between 40 and 70% by weight. Methods of upconcentration include for example thermal upconcentration, ultrafiltration as disclosed in U.S. Pat. No. 4,369,266 and decantation as disclosed in U.S. Pat. No. 2,037,953. Generally, the dispersions are up concentrated in the presence of a stabilizing surfactant such as a non-ionic surfactant.
In many applications, the PTFE dispersion resulting after polymerization and upconcentration are combined with further additives or components to produce a final composition. For example, in metal coatings, in particular for coating cookware, the final composition may be obtained by further blending heat resistant polymers such as polyamide imide, polyimide or polyarylen sulphide with the PTFE dispersion. Still further ingredients such as pigments and mica particles may be added as well to obtain the final coating composition for coating metal. Such additional components are typically dispersed in organic solvents such as toluene, xylene or N-methylpyrrolidone. The fluoropolymer dispersions typically represent about 10 to 80% by weight of the final composition. Coating compositions for metal coatings and components used therein have been described in e.g. WO 02/78862, WO 94/14904, EP 22257 and U.S. Pat. No. 3,489,595.
Problems may however arise in the preparation and/or application of the final coating composition to a substrate such as for example a metal substrate for cookware. For example, when spraying such a coating die clogging may occur after some time. Furthermore, coagulation may occur in the pumping system when pumping the dispersion to a coating station for coating for example glass fabrics. Still further, coagulation in the dispersion may occur when excess coating composition is removed by doctor blades. These problems are particularly noticeable when the PTFE dispersion used to prepare the final coating composition has a low amount of fluorinated surfactant. Such PTFE dispersions are nevertheless desirable from an environmental point of view.
Accordingly, it is desirable to overcome or at least to reduce the aforementioned problems. Desirably, the aforementioned problems are reduced or resolved without impairing or substantially impairing the good mechanical and physical properties of PTFE. Preferably the solution should be easy and convenient and should be cost effective and environmentally friendly.